It is known to couple two or more metal cavities loaded with dielectric resonators side-by-side to make a filter for a wireless microwave communication system. It is known to use two or more dielectric resonator loaded metal cavities coupled side-by-side to create a filter for a base station and repeater of a microwave communication system. It is also known to use elliptic function filter theory to design the above mentioned metal cavity filters for wireless communication systems that require a high rate of cutoff frequency response at both ends of the filter's transition band. The dielectric resonator loaded metal cavity filter only allows the resonate frequency of the resonators and its harmonics to pass through the filter and on to the output. The number of resonators used determines the characteristics of the passing signal, such as bandwidth, insertion loss, skirt response and spurious frequency response. The disadvantage to such filters is that the resonators not only allow the first harmonic of design frequency to pass, but also allow the other associated higher order harmonics of that frequency to pass through the filter. These higher order harmonics are known to interfere with other electronic devices.
FIG. 1 shows a typical cylindrical dielectric resonator used in a dielectric resonator loaded metal cavity filter. The materials for the resonator of FIG. 1 are usually dielectrics which are of a high quality factor and have a dielectric constant (K) somewhere between K=10 to 100. FIG. 2 is a top view depiction of the resonator showing its electric field lines at the lowest resonant mode. FIG. 3 is a side view depiction of the resonator showing its magnetic field lines at the lowest resonant mode. FIG. 4 shows a cylindrical metal cavity in a metal block. FIG. 5 shows the metal cavity of FIG. 4 loaded with a resonator and including a tuning screw. The resonator is shown includes a resonator support. FIG. 6 shows a detailed electric field distribution of a dielectric resonator loaded metal cavity. FIG. 7 shows a detailed magnetic field distribution of a dielectric resonator loaded metal cavity. The magnetic field of a resonator is perpendicular to the electric field of the resonator. The electric field is quite strong everywhere within equatorial plane of the resonator, except near the resonator center. Therefore, a cylindrical plug can be removed from the center of the dielectric resonator to receive the resonator support, without disturbing the electric field and the resonant frequency, significantly.
The stored electro-magnetic energy at a resonant frequency of one cavity can be transfer to another cavity through an aperture know as an IRIS in the cavity or by a conducting coupling probe, as shown in FIGS. 8-10 and 11-14. The output and input to a filter is usually radio frequency signals to and from an antenna or signal generator. The number of dielectric resonator loaded cavities used and coupling method between those cavities determine the characteristics of a filter. Such characteristics include bandwidth, insertion loss, skirt and spurious frequency responses of a filter. The disadvantage of dielectric resonator loaded metal cavity filters is that the resonating cavities not only allow the first harmonic of desired frequency to pass, but also allow other associated higher order harmonics of that frequency to pass through the filter. These higher order harmonics interfere with other electronic devices.
FIGS. 23-24 show an example of a six-pole dielectric resonator loaded metal cavity elliptic function filter, whereby all of the cavities are coupled side-by-side. FIG. 23 shows the filter with a cover removed and depicting how the cavities are coupled. All couplings between each cavity are positive side couplings including between a cavity and the input and output of the filter, except for the negative elliptic coupling of −k(2,5) and −k(1,6) that are shown. FIG. 24 is a top view of the filter which depicts a cover with tuning screws, where there is a tuning screw for each cavity. The positive side couplings between cavities are carried out by using a positive side coupling aperture, as shown in FIG. 8. The coupling between an input or output and a cavity, and between cavities where Negative Elliptic Coupling is employed are both accomplished by using a conducting coupling probe, as shown in FIGS. 11-12. The input/output connectors are usually N or SMA-type. The resonator support can be made from polymer or ceramic or polymer/glass fiber and/or polymer/ceramic composite materials, respectively. FIGS. 25-26 shows an example of a duplexer filter made up of two side-by-side filters, whereby each of the two filters is similar to the filter shown in FIGS. 23-24. FIG. 25 shows a perspective view with a cover removed and depicting how the cavities are coupled. FIG. 26 is a top view of the filter which depicts a cover with tuning screws.
It is an object of the present invention to provide a dielectric resonator loaded metal cavity filter that is more compact in nature.
It is another object of the present invention to provide dielectric resonator loaded metal cavity filter which filters out the higher harmonics associated with the desired frequency which is to pass through the filter.